The invention relates to a power supply arrangement comprising a DC/DC converter. Such a power supply arrangement can be used in power supplies or (switched-mode) power supplies or charging devices (for example for mobile radios), in which an AC mains voltage is converted by an AC/DC converter and by the DC/DC converter into a DC supply voltage.
Converters with asymmetric half-bridge topologies are basically known. Known converters have zero voltage switching of the half-bridge switch elements. The switch elements are controlled by pulse width modulated control signals with a constant frequency. An optimal resetting of the transformer core is guaranteed. The leakage inductance of the transformer is used. The power load of the switch elements is reduced compared to the otherwise often used blocking converters (flyback converters). The EMI load (Electro Magnetic Interference) by asymmetric half-bridge converters is small.
A DC/DC converter arranged as an asymmetric half-bridge converter is known from U.S. Pat. No. 5,808,879 (FIG. 9), comprising a half bridge which includes a first and a second switch element (switch elements arranged as MOSFET-transistors) to which a DC voltage is applied i.e. the half bridge is arranged between a first DC voltage potential (xe2x80x9c+xe2x80x9d supply potential) and a second DC voltage potential (xe2x80x9cxe2x88x92xe2x80x9d reference potential, customarily equal to the ground potential). A circuit branch which comprises the primary winding of a transformer and a capacitor connected in series to the primary winding is connected in parallel to the first switch element which is connected to the first DC voltage potential. The capacitor is arranged between the connection point of the two switch elements and the primary winding. The primary winding of the transformer is arranged between the capacitor and the first DC voltage potential. A capacitive output filter is arranged in parallel to the output of the DC/DC converter. Between the capacitive output filter and the secondary winding of the transformer is arranged a diode. In case of energy transport from the primary side to the secondary side of the converter, a current will flow on the primary side, on the one hand, through the circuit branch connected in parallel to the switched-off (opened) first switch element, which circuit branch includes the primary winding and the capacitor and, on the other hand, also through the switched-on (closed) second switch element.
The converter described in U.S. Pat. No. 5,808,879 is advantageous in that over-current protection is only necessary for the second switch element. Over-currents which lead to the destruction of a switch element may occur during the switch-on phase of the converter and in case of overload. When the converter is switched on, the capacitor connected in series to the primary winding of the transformer does not yet have charge. Accordingly, the current flowing through the second switch element by which the capacitor is charged strongly increases in the phase shortly after it has been switched on. Accordingly, the second switch element is to be protected against over-currents in the switch-on phase of the converter. The switch element that is switched on and through which a current flows during the energy transfer from the primary side to the secondary side of the transformer is to be protected from over-currents in the case of overload. With the converter described here it is the second switch element.
The converter described in U.S. Pat. No. 5,808,879 furthermore has the advantage that the so-termed burst mode is automatically set by measuring the input power on the primary side of the transformer. The burst mode is used with a reduced power consumption for a standby operation of a load and is characterized in that the converter is alternately switched off and on for a brief period of time. The converter is switched off especially when the two switch elements are simultaneously switched off. While the converter is switched off, the no-load losses of the converter are minimal. When the converter is switched on, the output capacitor is charged until the output voltage has reached its set value. This sequence of switching on and off causes a low power consumption with an available output voltage. The burst mode is only sensible with a small load, because otherwise the output voltage sharply drops during pauses of operation. For automatically activating and deactivating the burst mode, the input or output power of the converter can be measured. Measuring the input power is more cost effective compared to measuring the output power, because the electric isolation of output side and input side by the transformer need not be bridged. The input power is preferably measured by measuring the input voltage and the input current. The measurement of the input current is possible here simply by measuring the current flowing through the second switch element while only a measuring resistor (shunt resistor) is connected in series to the second switch element, which is so simple here because the second switch element is the only current path to the DC voltage potential coupled to the second switch element. It is particularly simple to measure the current when the measuring resistor is connected to the respective reference potential (ground potential) i.e. the voltage on the measuring resistor can be measured relative to the reference potential.
From the conference contribution by Phua Chee Heng, Ramesh Oruganti, xe2x80x9cFamily of Two-Switch Soft-Switched Asymmetrical PWM DC/DC Convertersxe2x80x9d, PESC 1994 is known a still older version of an asymmetrical half-bridge converter which is referred to as a buck reset TRC2-converter. This converter, however, does not have the two above-mentioned advantages of the converter of FIG. 9 of U.S. Pat. No. 5,808,879.
The converters described can be designed with or without a control of the output voltage. If a control is provided, the converter output voltage is customarily fed back via a feedback branch with an optocoupler, which leads to increased cost and is applied to a primary side control circuit which adapts the duty cycle of the control signals applied to the switch elements in dependence on the current converter output voltage, so that the output voltage is kept constant. Without a control of the output voltage, increased fluctuations of the converter output voltage especially in case of fluctuations of the input voltage and of the output current are to be taken into the bargain.
It would be advantageous to provide an asymmetrical half-bridge converter which has said advantages of the converter from U.S. Pat. No. 5,808,879, generates an output voltage with a smallest possible tolerance area, and is furthermore highly cost-effective.
Toward this end, a converter could be provided with a control loop on the primary-side of the transformer. Provided that a primary-side control loop is included, an optocoupler is not necessary for realizing a feedback path, so it could be eliminated to reduce cost. The primary-side control loop would be realized by circuit arrangement components located on the primary side.
In a primary-side control loop particularly the voltage which is impressed during the transmission of power from the primary side to the secondary side of the primary winding of the transformer is set to a predefinable fixed value, preferably by adapting the duty cycle of the control signals to be used for controlling the switch elements.
When operated in an uncontrolled manner the voltage on the primary winding and thus the output voltage of the DC/DC converter depending on it would fluctuate intolerably. Reasons for this are fluctuations of the input voltage, ripple of the input voltage and tolerances of the adjustable duty cycle based on production engineering tolerances of the control circuit which is particularly realized by an integrated circuit. To counteract fluctuations of the voltage on the primary winding or the converter output voltage, respectively, the primary-side control is used. The control signals to be adjusted for the primary-side control for controlling the switch elements are particularly pulse width modulated i.e. an adjustment of the control signals is made by adjustment of the respective duty cycle. Alternative embodiments of the invention could utilize an additional capacitor.
The power supply arrangement according to an embodiment of the invention can be arranged for obtaining a measuring value for the voltage on the primary winding when the DC/DC converter is switched on by the tapping of the potential between the primary winding and the capacitive element. Since the second switch element coupled to the second DC voltage potential is turned on during energy transfer, and the capacitive element is coupled to the first DC voltage potential, this potential between the capacitive element and the primary winding, irrespective of the instant in a switching cycle, represents the voltage on the primary winding of the transformer during energy transfer from the primary side to the secondary side, so that the converter output voltage is set with a sufficient tolerance for many applications (for example, charging devices for mobile telephones) in this manner. Basically, the arrangement also works with a primary-side control when there is an energy transfer from the primary side to the secondary side in the other state of the half bridge or when there is fill-wave rectifying in the two states of the half bridge.
Various embodiments of the power supply arrangement according to the invention are possible, including use of inductive output filters and/or full-wave rectifying. At least one circuit arrangement according to the invention disclosed herein is well-suited, for example, for use in a charging device for a mobile telephone.
These and other aspects of the invention are apparent from and will be elucidated with reference to the examples of embodiment described hereinafter.